GLACIAL SCULPTURE 
207 
The importance of plucking may also be inferred, as in 
fact it has been by Dana, from the abundance of boulders 
in the moraines of an ice-sheet. In the waste deposited 
by an alpine glacier it is not easy to discriminate 
plucked boulders from the boulders which have fallen to 
the ice from adjacent rock slopes and been carried for¬ 
ward as back-load, but the waste carried to the edge of a 
great ice-sheet like the Laurentide has all been picked 
up as well as transported by the ice. A portion of such 
a body of waste must be referred to the mantle of resid¬ 
uary and alluvial debris found initially on the land by the 
expanding ice mass, and this portion of course includes a 
contingent of boulders; but there is no reason to regard 
this factor as of great importance. It is probably much 
more than offset by the destruction of boulders in the 
glacial mill, for in the making of the rock flour which 
constitutes the body of glacial till, the abrasion of the 
coarser waste carried by the bottom ice may approach, 
or even exceed, the abrasion of the rock floor. While 
these various factors do not admit of definite valuation, I 
think it fair to say that the ratio of boulders, on the one 
hand, to clay and sand, on the other, in the waste deposits 
as a whole, is something less than the ratio of plucking 
to abrasion in the erosive work of such an ice-sheet as 
the Laurentide. 
The study of this subject made such slow progress in 
the field that opportunities for good photographic illustra¬ 
tion were not improved. The views reproduced in plate 
xvin were taken to show abrasive work, and illustrate 
plucking only incidentally. The upper view looks up the 
wall of Muir Inlet. It shows stratified rock, with a boss 
of more massive, possibly plutonic, rock beyond. A 
century ago all the stratified rock was covered by Muir 
Glacier. At the left near the sky-line the strata are seen 
to be obliquely truncated, and the plane or nappe of trun- 
